Major, trace element, and isotopic study of the Lonar Crater, India: new insights into the geochemical imprint of impact cratering on basaltic targets

English Abstract:

The 1.88 km diameter Lonar crater, hosted on the Deccan basalts, India, has age estimates ranging from 50-500 ka. Being a rare terrestrial impact crater on basalts, it provides a unique opportunity to study the geochemical imprint of impact cratering on basaltic host rocks which is relevant to other terrestrial planets, the Moon, as well as asteroids. We measured concentrations of 43 elements using ICPMS in recently collected samples of the target Deccan basalt, impact melts and spherules at Lonar. Compared to the basalt, the impact melts (tektites) are enriched in Rb, K, Ba, Pb, Mn, Cr, U, Th, as well as light REEs (La, Ce, Pr, Nd). The spherules broadly show similar geochemical characteristics as the melts. In addition, the spherules are depleted in P, enriched in Cr, Ni, and some heavy REEs like Tm and Lu, but markedly depleted in Mn compared to the target basalts. For most of these elements, the difference in concentrations between the target basalts and impactites is more than 20% and as high as 100%. We have also measured Sr and Nd isotopic compositions of the basalts, melts and spherules using TIMS and our data overlap with previously published data for the Lonar rocks that argued for melting of the Archean basement beneath the Deccan basalts during impact. The impact melts as well as spherules show more radiogenic Sr and less radiogenic Nd isotopic composition compared to the target basalts. The Sr isotopic composition co-varies with the Rb/Sr as well as K/Ca ratios while the Nd isotopic composition shows a positive correlation with the Sm/Nd ratio. Enrichments of Rb, Ba, K, Th, U, Pb and LREEs in the melts and spherules can be explained by mixing with Archean continental crustal rocks beneath the Deccan basalts. The Sr and Nd isotopic variations are also consistent with this explanation. However, enrichments in Cr, Ni compared to the target basalts are indicative of mixing with the impactor.